Commit 3567fa63 authored by Ard Biesheuvel's avatar Ard Biesheuvel Committed by Catalin Marinas
Browse files

arm64: kaslr: Adjust randomization range dynamically 



Currently, we base the KASLR randomization range on a rough estimate of
the available space in the upper VA region: the lower 1/4th has the
module region and the upper 1/4th has the fixmap, vmemmap and PCI I/O
ranges, and so we pick a random location in the remaining space in the
middle.

Once we enable support for 5-level paging with 4k pages, this no longer
works: the vmemmap region, being dimensioned to cover a 52-bit linear
region, takes up so much space in the upper VA region (the size of which
is based on a 48-bit VA space for compatibility with non-LVA hardware)
that the region above the vmalloc region takes up more than a quarter of
the available space.

So instead of a heuristic, let's derive the randomization range from the
actual boundaries of the vmalloc region.

Signed-off-by: default avatarArd Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20231213084024.2367360-16-ardb@google.com


Signed-off-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
Acked-by: default avatarMark Rutland <mark.rutland@arm.com>
parent d432b8d5

arch/arm64/kernel/image-vars.h

+2 −0
Original line number Diff line number Diff line
@@ -36,6 +36,8 @@ PROVIDE(__pi___memcpy = __pi_memcpy); 
PROVIDE(__pi___memmove			= __pi_memmove);

PROVIDE(__pi___memset			= __pi_memset);



PROVIDE(__pi_vabits_actual		= vabits_actual);



#ifdef CONFIG_KVM



/*

arch/arm64/kernel/pi/kaslr_early.c

+6 −5
Original line number Diff line number Diff line
@@ -14,6 +14,7 @@ 


#include <asm/archrandom.h>

#include <asm/memory.h>

#include <asm/pgtable.h>



/* taken from lib/string.c */

static char *__strstr(const char *s1, const char *s2)
@@ -87,7 +88,7 @@ static u64 get_kaslr_seed(void *fdt) 


asmlinkage u64 kaslr_early_init(void *fdt)

{

	u64 seed;

	u64 seed, range;



	if (is_kaslr_disabled_cmdline(fdt))

		return 0;
@@ -102,9 +103,9 @@ asmlinkage u64 kaslr_early_init(void *fdt) 
	/*

	 * OK, so we are proceeding with KASLR enabled. Calculate a suitable

	 * kernel image offset from the seed. Let's place the kernel in the

	 * middle half of the VMALLOC area (VA_BITS_MIN - 2), and stay clear of

	 * the lower and upper quarters to avoid colliding with other

	 * allocations.

	 * 'middle' half of the VMALLOC area, and stay clear of the lower and

	 * upper quarters to avoid colliding with other allocations.

	 */

	return BIT(VA_BITS_MIN - 3) + (seed & GENMASK(VA_BITS_MIN - 3, 0));

	range = (VMALLOC_END - KIMAGE_VADDR) / 2;

	return range / 2 + (((__uint128_t)range * seed) >> 64);

}